Metronidazole is a drug that is widely used for treatment of anaerobis bacterialinfections protozoa in humans and animals. It has also been used to radiosensi tize hypoxic mammalian cells. Though the mode of action is not clear, it is contingent on the reduction of the compound to reactive intermediate species. We hypothesize that nitric oxide contributes directly or indirectly to the cidal action of this compound. Our objectives are to demonstrate the in vitro and in vivo production of nitric oxide following reduction of metronide, and to characterize the cellular targets of nitric oxide in the anaerobis protozoa Trichomonas fetus.